DNA can be damaged by a variety of environmental insults, including antitumor drugs, radiation, carcinogens, mutagens and other genotoxins. Chemical changes in the component nucleotides or of DNA secondary and tertiary structure which arise from such external causes are all considered herein to be DNA modification or damage. In addition, it is recognized that certain chemical and/or structural modifications in DNA may occur naturally, and may play a role in, for example, DNA replication, expression, or the coordinate regulation of specific genes. It has been proposed that some types of DNA modification or damage arising from external sources are similar to, or even mimic, certain types of natural DNA chemical and/or structural modification.
DNA damage can lead to mutations and cancer, as well as cell death; the latter is exploited in chemo- and radio-therapeutics. A better understanding of DNA chemical and structural modifications, including DNA damage, would also be helpful in that it might serve as the basis for developing an enhanced ability to repair or otherwise modify the effects of such damage, leading in turn to improved organismal or tissue resistance to DNA damaging agents.